One of the issues to be solved in developing the Wake Turbulence Timers was bezels for the red and yellow 3 digit 7-segment LED displays. The old-school traditional type clamped to the panel and provided long 4-40 threaded rods to support the printed circuit board in the proper position. There are two problems with these - they only have red filters and they are over twenty bucks a piece!
I received 3 of these meters today. The board is held in by a tab on each end and can be pried out without any problem.
This photo shows the housing, the lens and the removed meter circuit board. Wow, there is almost no clearance around the 3 digit display on the circuit board! This is a multiplexed display, so few traces are needed on the circuit board. I have to use 3 individual digits - the second one is rotated so the decimal points form a colon. M:SS
Here are the dimensions of a board that will fit. I need to verify that the digits I have will actually fit!
So in my ever-so-slow progress, I located 3 of the digits I use for the Wake Turbulence Timers; they do fit with perhaps a millimeter to spare in each direction.
The boards originally in these units used a 3 digit display, meaning there are just 11 connections to be made to the driver chip. For my application, I need to use individual digits, as I rotate the second one to use the decimal points as a colon. That means 8 connections in parallel across the digits and the three common connections for the digits.
My plan is to use a MAX7219 as the driver. I need a 6 pin header for the I2C signals and ideally a second one to daisy-chain at least 2 modules.
The MAX7219 has the ability to decode the digits you send. For example, if you send an ASCII 7, the chip will decide what segments need to be illuminated to display the digit 7. To make this work, I need to remap the pinout for the rotated digit. Segment G and the decimal points swap positions when the digit is rotated. I had hoped to include some solder jumpers to select a normal or rotated digit but the tiny board size may preclude this. The display will still be usable with the digit in either orientation, but in one position I can't use the built-in decode feature.